src/Ganeti/SlotMap.hs

{-| A data structure for measuring how many of a number of available slots are
taken.

-}

{-

Copyright (C) 2014 Google Inc.
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CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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-}

module Ganeti.SlotMap
  ( Slot(..)
  , SlotMap
  , CountMap
  , toCountMap
  , isOverfull
  , occupySlots
  , hasSlotsFor
  ) where


import Data.Map (Map)
import qualified Data.Map as Map

{-# ANN module "HLint: ignore Avoid lambda" #-} -- to not suggest (`Slot` 0)


-- | A resource with [limit] available units and [occupied] of them taken.
data Slot = Slot
  { slotOccupied :: Int
  , slotLimit    :: Int
  } deriving (Eq, Ord, Show)


-- | A set of keys of type @a@ and how many slots are available and (to be)
-- occupied per key.
--
-- Some keys can be overfull (more slots occupied than available).
type SlotMap a = Map a Slot


-- | A set of keys of type @a@ and how many there are of each.
type CountMap a = Map a Int


-- | Turns a `SlotMap` into a `CountMap` by throwing away the limits.
toCountMap :: SlotMap a -> CountMap a
toCountMap = Map.map slotOccupied


-- | Whether any more slots are occupied than available.
isOverfull :: SlotMap a -> Bool
isOverfull m = or [ occup > limit | Slot occup limit <- Map.elems m ]


-- | Fill slots of a `SlotMap`s by adding the given counts.
-- Keys with counts that don't appear in the `SlotMap` get a limit of 0.
occupySlots :: (Ord a) => SlotMap a -> CountMap a -> SlotMap a
occupySlots sm counts = Map.unionWith
                          (\(Slot o l) (Slot n _) -> Slot (o + n) l)
                          sm
                          (Map.map (\n -> Slot n 0) counts)


-- | Whether the `SlotMap` has enough slots free to accomodate the given
-- counts.
--
-- The `SlotMap` is allowed to be overfull in some keys; this function
-- still returns True as long as as adding the counts to the `SlotMap` would
-- not *create or increase* overfull keys.
--
-- Adding counts > 0 for a key which is not in the `SlotMap` does create
-- overfull keys.
hasSlotsFor :: (Ord a) => SlotMap a -> CountMap a -> Bool
slotMap `hasSlotsFor` counts =
  let relevantSlots = slotMap `Map.intersection` counts
  in not $ isOverfull (relevantSlots `occupySlots` counts)